Defrosting control for refrigerating system



Dec. 27, 1955 F. R. ELLENBERGER g I 2,728,19?

DEFROSTING CONTROL FOR REFRIGERATING' SYSTEM v Filed Sept. 18, 1,952

lhventOY Francis R EHenberger,

His Attorney.

United States Patent DEFROSTING CONTROL FOR REFREGERATING SYSTEM FrancisR. Ellenberger, Verona, N. 5., assignor to General Electric Company, acorporation of New York Application September 18, 1952, Serial No.310,308

3 Claims. (Cl. 62- 4) This invention relates to refrigerating systemsand particularly to systems having evaporator coils for cooling air andcontrols for effecting defrosting of the coils automatically.

Refrigerating systems for cooling air frequently include evaporators orcooling units which must operate below the freezing point of water andon which substantial quantities of frost collect and decrease the rateof heat transfer, thereby decreasing the effectiveness of the system. Itis desirable to provide some arrangement for effecting defrostingautomatically without the necessity of frequent attendance of anoperator and manual servicing. The defrosting operation may beaccomplished in many ways; for example, operation of the cooling unitmay be discontinued a sufficient length of time to permit a rise intemperature and melting of the frost, or heat may be appliedartificially. The heat may be supplied either directly at the coolingunit or by heating the air circulated over the unit or by reversing therefrigerating machine to supply hot compressed refrigerant to thecooling unit. Reversed cycle refrigerating machines may be employed asyear-round air conditioning equipment, and during the winter seasonfrost collects on the outdoor heat transfer coil. It is desirable thatair conditioning systems require minimum attention, and it is thereforedesirable to provide defrosting automatically whenever a predeterminedaccumulation of frost has collected on the cooling unit. Accordingly, itis an object of this invention to provide a refrigerating machine havinga cooling unit for cooling air to temperatures below freezing and animproved automatic defrosting control.

It is another object of this invention to provide a defrosting controlfor refrigerating systems including a simple and effective arrangementfor initiating and terminating defrosting automatically.

Further objects and advantages of the invention will become apparent asthe following description'proceeds, and the features of novelty whichcharacterize the invention will be pointed out with particularity in theclaims annexed to and forming apart of this specification.

In carrying out the objects of this invention, a refrigerating machineis provided in an air cooling system including a cooling unit arrangedin an air duct having a blower for circulating air over the surface ofthe unit. In order to indicate the accumulation of frost 0n the coolingunit, a small by-pass is provided to circulate air around the coolingunit and a flow responsive device is arranged in the by-pass to initiatedefrosting whenever the flow of air through the by-pass is increased toa predetermined amount, this increase being caused by the accumulationof frost on the cooling unit and the consequent restriction of the mainair duct. The flow responsive device may be employed to initiatedefrosting by reversing the refrigerating machine to supply heat to thecooling unit.

For a better understanding of the invention, reference may be had to theaccompanying drawing the single figure of which illustratesdiagrammatically a heat pump installation for air conditioning providedwith a control for automatically defrosting the outdoor air heatexchanger.

Referring now to the drawing, the air conditioning system illustratedincludes an indoor heat transfer coil 10 arranged in a duct 11 throughwhich air to be conditioned is circulated by a blower 12 driven by amotor 13, return air being admitted to the duct 11 through an inlet 14and fresh air through an inlet 15, and conditioned air being returned tothe rooms through an outlet 16. The heat transfer unit 10 may beconnected either as the evaporator or the condenser of a refrigeratingmachine so that the air in the duct 11 may be either heated or cooled asdesired. The refrigerating machine includes a refrigerant compressor 17driven by an electric motor 18 and an outdoor heat transfer unit 19, thetwo heat transfer units and the compressor being connected in a closedrefrigerant circuit provided with threeway reversing valves 20 and 21for selecting either the heating or the cooling operation. The heattransfer unit 19 is arranged in an outdoor air duct 23 arranged toreceive outdoor air from an inlet 24 in the wall of the buildingindicated at 25 and provided with a blower 26 driven by a motor 27 forcirculating the air through the duct and returning it to the outsidethrough a duct connection (not shown). The system is controlled byoperation of a bi-metallic blade thermostat 28 arranged to engage aheating contact 30 when the room temperature is below a predeterminedvalue and a cooling contact 29 when the temperature is above a higherpredetermined yalue. The electrical control system is connected to thesecondary of a transformer 35 the primary of which is connected acrosssupply lines 31 and 32, and upon operation of the thermostat 28 either acooling relay 33 or a heating relay 34 is operated. Each of these relaysis provided with two switches, the lower of each of which connects acoil 36 of a solenoid switch 37 across the lines 31 and 32 and energizesthe motors 13, 18 and 27, thereby starting the operation of the airconditioning system.

During the cooling season, the refrigerating machine operates to supplyhot compressed refrigerant to the outdoorheat exchange unit 19 throughthe valve 21, and the indoor heat exchange unit 10 is connected as anevaporator to cool the air circulated through the duct 11. Thisoperation is effected upon movement of the thermostat 28 to engage theleft-hand contact 29 and actuate the cooling switch 33, to close thenormally open upper contacts thereof. The upper contacts of the switch33 completea' circuit from the line 31 through a connection 38, then inparallelthrough lower solenoid coils 39 and 40 of the three-way: valves20 and 21, respectively, and thence back to the'line 32 through aconnection 41. The compressor 17 then operates to discharge hotcompressed refrigerant to the valve 21, thence through the lower outletof the valve and a conduit 42 to the unit 19 where the refrigerant iscooled and liquefied upon giving up heat to the air circulated by theduct 23. The liquefied refrigerant then flows through a connection 43and a check valve 44 to a liquid receiver 45. From the liquid receiver,the refrigerant flows through a liquid line 4-6 under control of athermostatic expansion valve 47 to the unit 10 through a connection 48.The liquid refrigerantin the unit 10 is vaporized by the absorption ofheat from the air flowing through the duct 11, and the vaporizedrefrigerant is returned to the compressor through the upper inlet of thevalve 20 and a suction line .49 connecting the outlet of the valve 20and the intake of the compressor.

During the heating season when the temperature falls to .a predeterminedvalue below the range of operation- Patented Dec. 27, 19 55 during. thecoolingv season, the bimetallic blade thermostat 28 engages the contact30 and energizes the coil of the switch 34, this coil being connectedacross the secondary of the. transformer through the thermostat and anormally closed set of contacts of a. solenoid switch-51 having foursets of contacts 50, 52,53 and 54 respectively. Upon energization of theswitch 34, its lower contacts close the circuit of the coil 36 toactuate the switch 37 and initiate operation of the refrigeratingmachinev and air circulating system. The upper set of. contacts of theswitch 34 energizes the circuit of actuating solenoids 55 and 56 of thevalves 20 and 21. This: circuit may be traced. from the line 31 throughthe upper contacts of the switch 34, a connection 57, the coils 55 and56in parallel, and thence to the line 32 through the connection 41.Energization of the coils 55 and 56 shifts the valves 20 and 21 so thatthe compressor 17 delivers hot compressed refrigerant to the unit 10through connection 48, the thermostatic valve 47 automatically closingto prevent passage of refrigerant into the line 46. The refrigerant isliquefied in the unit 10 by the cooling of the. air passing through theduct. The air is thereby warmed and returned to the rooms. The liquefiedrefrigerant then flows through a line 58 and a check valve 59 to thereceiver 45 and then flows from the receiver through the line 46 to theunit 19 under control of a thermostatic expansion valve 60. Therefrigerant in the unit 19 is vaporized by the absorption of heat fromthe outdoor air flowing through the duct 23, and the vaporizedrefrigerant is returned to the compressor through a suction con nectioncomprising the line 43, a connection 61, the valve 20, and the suctionconnection 49. The operation of. the system continues to eflect heatingof the air in the rooms supplied by operation of the blower 12 until thethermostat 28 reaches a predetermined high temperature at which it movesaway from contact 30 and stops operation of the system.

During the operation of the refrigerating machine to supply heat to theair through duct 11, it is necessary at times to operate the unit 19 asan evaporator below the freezing point of water, and the moisturecondensed on the surface of the unit 19 is frozen and builds up a layerof frost. In order to maintain the efiicient operation of the, unit 19,it is necessary to remove the frost before it accumulates to an extentthat heat transfer is seriously impeded. In order to detect theaccumulation of frost, an air duct 62 is provided constituting a by-passaround the unit 19. The duct 62 receives air through an; inlet 63 andreturns the air to the duct 23 through an outlet 64. As frostaccumulates on the surface of the unit 19, the flow of air through theunit is decreased by. the de creased effective cross-sectional area ofthe duct and an increased flow of air occurs through the by-pass 62.Thisincreased flow of air may be employed. as an indication of thepresence of a predetermined accumulation of. frost on the surface of theevaporator 19, and, for this purpose, a device dependent upon the changeof the flow Off air in the duct 62 is employed to initiate thedefrosting operation of the system.

In the system illustrated in the drawing, a snap-acting switch 65 isarranged to be actuated by a vane 66mount ed in the by-pass 62. The vaneis normally biased to a position toward the intake 63 of the by-pass .byan. adjustable compression spring 67. The vane is therefore normally ina position opposing the flow of air from the inlet 63 toward the outlet64. As the passage of air through the duct 23 is restricted by theaccumulation of frost on the unit, increased air flows through by-pass62 until the force thereof is sufficient to move the vane 66' to theleft about its pivot indicated at 68 and thereby compress the spring 67and move an'arm 6.9 of the switch 65 to the right: until. a spring70.passesovercenten with respect to a pivot 71 and causes actuation ofav switch arm 72. The-switch arm 72 normally restsagainst a stop. 73and: upon actuation is snapped bysthe'. over:

center. action. Of the spring 70 to engage a contact 74. This closes acircuit from one side of the secondary of the transformer 35 through aline 75, the switch arm 72, and contact 74, and thence through a line76, a coil 77 of the switch 51, and back to the other side of theprimary through a connection 78. The coil 77 is thereby energized andactuates the switch 51 to open its contacts 50 and thereby preventenergization of the coil of the switch 34 and close its contacts 52 toenergize the switch 33. It is thus apparent that the operation of therefrigerating machine will be reversed by de-energizing of the coils 55and 56 and energizing the coils 39 and 40. Thereupon, the hot compressedrefrigerant will be delivered to the unit 19 through the connection 42and the heat will melt the frost on the surface thereof, the resultingwater being drained off through any suitable connection, not shown.

During this defrosting operation, circulation of cold outdoor airthrough the duct 23 is prevented by stopping the blower motor 27 thecircuit of which is opened when the coil 77 is energized to actuate theswitch 51, the normally closed contacts 53 being in the circuit of themotor 27. At the same time the normally open contacts 54 are closed toshunt the vane actuated switch 65,

through a lead 78 and a switch 79 actuated by a temperature responsivebellows 80. Thus, even though the vane 66 is restored to its normalposition, separating the contacts 72 and 74 due to interruption ofairflow through duct 23 when blower motor 27 is stopped, thev defrostingoperation will be continued because the coilv 77 will be energizedthrough the shunt or holding cit:- cuit. The temperature responsivebellows 80 is provided with a temperature feeler element 81 adjacent theoutlet.

line 43 of the coil 19, and when the, temperature of the refrigerantwithdrawn from the coil exceeds a predetermined value the, bellows 80expands and opens the switch 79, thusv terminating the defrostingoperation, the switch 65 being open at this time since it will have beenrestored by the stopping of air flow through the duct 62 when contacts53 are opened. Deenergization. of the switch coil 77 opens contacts 52and 54 and closes.

contacts 50 and 53 thereby deenergizing the coil of the switch 33 andenergizing the coil of the switch 34' provided there is a call forheating by the thermostat 28.

The temperature responsive bellows 80 and holding circuit includingswitch 79 have been illustrated byway of example, it being obvious tothose skilled in the art that other mechanisms may be employed tocontinue the defrosting operation for the desired period.

The degree of frosting of the evaporator necessary to actuate the switch65 may be adjusted by adjustingv the. compression of the spring 67 byturning of, a knob 82. Theamount of air flowing through the duct 62 mayalso be variedto adjust the setting of the control by varying the.position. of adamper 83 arranged. to restrict the passage through the.inlet 63. The damper 83 may be adjusted by turning its threaded stem,indicated at 84.; From the foregoing, it is readily apparent that a.-

simple and effective arrangement has been provided for initiatingdefrosting of the evaporator of a refrigerat ing machine automaticallyupon the accumulation of a predetermined amount of frost on its surface.Furthermore, the amount of accumulation of frost which is to bepermitted may readily be determined by adjust- What I claim as new anddesire to secure by Letters Patent of the United States is:

1. A refrigerating machine including a cooling unit, means forcirculating over said unit air to be cooled, means for operating saidmachine to cool said unit below the freezing point of Water wherebyfrost tends to collect on the surface of said unit, duct means providinga by-pass around said unit whereby an increased rate of flow of airoccurs through said by-pass upon restriction of the path of flow throughsaid unit upon the accumulation of frost thereon, a movable vane in saidby-pass in the path of the air flowing therethrough, means for biasingsaid vane to a position toward the inlet of said by-pass, meansdependent upon movement of said vane away from said position foreffecting the defrosting of said unit, means responsive to thetemperature of the refrigerant in said cooling unit to terminate thedefrosting operation, and damper means in said by-pass for varying theeffective inlet area of said by-pass for determining the degree offrosting of said coil required to actuate said vane.

2. A refrigerating machine including a compressor and a condenser and anevaporator connected in a closed refrigerator circuit, means forcirculating air to be cooled over said evaporator, means for operatingsaid machine to cool said evaporator below the freezing point of waterwhereby frost tends to collect on the surface of said evaporator, meansincluding a switch for reversing the refrigerant circuit of said machineto interchange the functions of said evaporator and said condenser andsupply heat to said evaporator to defrost the surface thereof, ductmeans providing a by-pass for conducting air around said evaporatorwhereby an increased rate of flow of air occurs through said by-passupon a restriction of the path of flow through said evaporator upon theaccumulation of frost, a movable vane in said by-pass connected toactuate said switch, means for biasing said vane in a directionresisting the flow of air through said by-pass whereby upon theoccurrence of a predetermined fiow of air through said by-pass said vaneactuates said switch to initiate defrosting of said evaporator, andmeans responsive to the temperature of the evaporator during defrost toterminate the defrosting operation.

3. A refrigerating machine including a compressor and a condenser and anevaporator connected in a closed refrigerant circuit, means forcirculating air to be cooled over said evaporator, means for operatingsaid machine to cool said evaporator below the freezing point of waterwhereby frost tends to collect on the surface of said evaporator, meansfor reversing the refrigerant circuit of said machine to interchange thefunctions of said evaporator and said condenser and supply heat to saidevaporator to defrost the surface thereof, duct means providing aby-pass for conducting air around said evaporator whereby an increasedrate of flow of air occurs through said by-pass upon a restriction ofthe path of flow through said evaporator upon the accumulation of frost,means including an electrical switch for reversing said refrigeratingmachine to effect defrosting of said evaporator, a movable vane in saidby-pass connected to actuate said switch, means for biasing said vane ina direction resisting the flow of air through said by-pass whereby uponthe occurrence of a predetermined flow of air through said by-pass saidvane actuates said switch to initiate defrosting of said evaporator,means responsive to the temperature of the refrigerant in said coolingunit to terminate the defrosting operation, and damper means foradjusting the effective inlet area of said by-pass to determine thedegree of restriction of air through said evaporator necessary toinitiate defrosting.

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